1. Field of the Invention
The present invention relates generally to electrical circuits, and more particularly to circuit elements for generating a clock to within the required accuracies of a frequency.
2. Description of the Background Art
An accurate clock is often generated by a crystal oscillator. Other clocks are then generated from this accurate clock. However, the size of the crystal oscillator can be bulky. In most of the portable products, the size of the product needs to be minimized. Furthermore, the cost of a crystal oscillator is relatively expensive if the oscillation frequency of the crystal oscillator is high. In contrast, the size of a clock generator built upon the very large scaled integrated circuits is small and can be very cost effective. The challenge lies on how to maintain frequency accuracy over process, voltage, and temperature (PVT) variations.
A periodic signal such as a clock can be generated by oscillators in the very large scaled integrated circuits, which can be either a ring oscillator or a LC-tank oscillator. In general, the type of LC-tank oscillators shows less frequency changes over PVT variations than the type of ring oscillators does. The oscillation frequency of both types of oscillators can be adjusted by changing the capacitances of their tuning circuit elements whose capacitances depend on the values of the associated control signals. The tuning circuit elements can be a variety of components and circuits, such as transistors or varactors. The capacitance of a tuning circuit element can be changed in a digital way or in an analog way. When tuned in a digital way, the control input of a tuning circuit element, being a binary one or a binary zero, enables or disables the tuning circuit element, respectively, to exhibit a larger capacitance or a smaller capacitance. When tuned in an analog way, the value of the control input of a tuning circuit element, being an analog voltage, determines the capacitance of the tuning circuit element.
To generate an oscillator's clock to within the required accuracies of a specified frequency against PVT variations, the frequency of the oscillator's clock can be measured for each operation point over PVT corners and then be adjusted by its tuning circuit elements accordingly. However, this process requires multi-point calibrations, which might be impractical in terms of the testing cost.
The frequency of the accurate clock is adjusted to the specified frequency at a known supply voltage and a known temperature during a one-point calibration. After the one-point calibration, the frequency changes of the accurate clock from the specified frequency due to process variations are eliminated to a negligibly small amount. However, the frequency of the accurate clock is only accurate at the known supply voltage and temperature. As the supply voltage and on-chip temperature change, the frequency of the accurate clock can deviate from the specified frequency.
To maintain the frequency accuracy against the variations of the on-chip temperature, a temperature sensor is employed to measure the on-chip temperature. The measured temperature is coupled to the input of a temperature compensated frequency controller, which estimates the frequency change of an accurate clock as the temperature changes. The frequency of the accurate clock can be adjusted by two methods. The first method is to change the capacitances of oscillators' tuning circuit elements. The second method is to use a fractional-N phase-locked loop, and then adjust the frequency of its output clock by changing its associated fractional control word. It is well known that the frequency of the output clock of a fractional-N phase-locked loop is equal to the multiplication of the frequency of its input clock and the value of the fractional control word. Therefore, adjusting the value of the fractional control word can change the frequency of the output clock. The output clock is then the accurate clock.
To maintain the frequency accuracy against the variations of the supply voltage, a linear regulator is usually employed to maintain the same supply voltage as the known supply voltage used during the one-point calibration. To further minimize the frequency changes, a digitally controlled oscillator can be employed. The digitally controlled oscillator employs digitally controlled tuning circuit elements. A digitally controlled tuning circuit element is enabled and disabled by setting its binary control signal to a binary one and a binary zero, respectively. Because of the binary nature of the control signal, the oscillation frequency of the digitally controlled oscillator is quite insensitive to the variations of the supply voltage.